1. Field of the Invention
The present invention relates to an ATM virtual path switching node which switches a virtual path influenced by various faults generated in an asynchronous transfer mode (to be simply referred to as ATM hereinafter) network to another virtual path as a detour, thereby recovering the path from the faults.
2. Description of the Prior Art
In recent years, as the bandwidth has broadened along with the progress in the optical fiber transmission technology, a broadband ISDN (BISDN) which integrally accommodates various communication services for voice, character, and image data and provides these services to subscribers is becoming more important. An essential network technology for realizing the BISDN is the ATM technology. In ATM, any information is segmented into packets called cells each having a fixed length, and a header necessary for routing is added to each packet, thereby transmitting the information, as described in Hiroshi Shimizu & Hiroshi Suzuki, "ATM-LAN", Soft Research Center. In this case, the cell is transmitted to a receiving ATM node or terminal on two logical connections, i.e., a virtual circuit and a virtual path.
The virtual circuit is a connection set for every call generated by assigning network resources (a route and a band) necessary for communication among a plurality of terminals in accordance with signaling procedures.
The virtual path is a logical transmission line which is semi-permanently set in advance between predetermined nodes in advance in accordance with the predicted traffic demand for between nodes such as switching systems or transmission apparatuses in the ATM network or the result of monitoring the actual amount of traffic. The virtual path does not depend on the network topology formed by actually connecting transmission lines such as optical fibers and coaxial cables. The virtual path accommodates a plurality of virtual circuits.
As a conventional method of recovering the virtual path from a fault generated in the ATM network, an ATM virtual path switching method is used (Yoshitaka Fujita et al., "ATM VP Protection Switching and Applications", Proceedings of International Switching Symposium, P.g20, pp. 82-86, April, 1995).
FIG. 1 is a view for explaining the conventional ATM virtual path switching method.
In FIG. 1, ATM nodes A to D are connected via links 600 to 607. A signal is transferred counterclockwise in the links 600 to 603, and clockwise in the links 604 to 607. Links 608 and 609 are connected to the nodes B and D, respectively.
A virtual path 610 is set in the links 600 and 603 between the counterclockwise nodes B and D. The node D transmits a cell on a virtual path 630 in the link 609 to the node B through the virtual path 610. The node B receives the cell on the virtual path 610, which is transmitted from the node D, and outputs it onto a virtual path 640 in the link 608. The upstream and downstream terminal nodes of the virtual path 610 are the nodes D and B, respectively. For the virtual path 610, a stand-by virtual path 620 to which the same band as that of the virtual path 610 is assigned is set in the clockwise links 605 and 606 in advance, thereby preparing for any fault in the network.
A method of detouring a cell transmitted through the virtual path 610 to the stand-by virtual path 620 when faults have occurred in the links 603 and 607 will be described next. When a fault occurs in the link 603, the node A detects the fault in the link 603 and determines that the fault will cause a fault in the virtual path 610. The node A as a fault detection node of the link and virtual path notifies the node B as the downstream terminal node of the virtual path 610 of the fault in the virtual path 610.
Upon being notified of the fault in the virtual path 610 by the node A, the node B notifies the node D as the upstream terminal node of the virtual path 610 of the fault in the virtual path 610.
Upon being notified of the fault in the virtual path 610 by the node B, the node D switches the virtual path 610 to the stand-by virtual path 620 to detour the cell from the virtual path 630 in the link 609 connected to the virtual path 610 to the stand-by virtual path 620. Next, via the virtual path 620, the node D notifies the node B of completion of switching from the virtual path 610 to the virtual path 620.
When the node D notifies the node B of completion of virtual path switching, the node B switches the virtual path connected to the virtual path 640 in the link 608 from the virtual path 610 to the virtual path 620 and outputs the cell from the virtual path 620 to the virtual path 640 in the link 608.
As described above, conventionally, when a fault occurs in the link 603, the fault is detected in units of virtual paths in the link 603. The upstream and downstream terminal nodes D and B of the virtual path are notified of the fault, and the virtual path 610 is switched to the stand-by virtual path 620.
Conventionally, since the fault detection means is arranged in units of virtual paths, fault detection and switching to a stand-by virtual path are independently performed in units of virtual paths. In one link, only a faulty virtual path is switched without affecting the remaining virtual paths having no fault.
However, the first problem of the prior art is that when faults are detected from a number of virtual paths, the processing load for generation or transmission of link fault notification signals increases, and accordingly, the virtual paths take a long time to recover from the faults.
The reason for this is as follows. The terminal nodes must be notified of faults in units of virtual paths. Even for virtual paths sharing the upstream or downstream terminal node, link fault notification signals corresponding to the number of the faulty virtual paths occur at the fault detection node and transmitted to the terminal node.
The second problem is as follows. When fault occur, and the number of virtual paths which must be switched is large, the processing load for generation and transmission of control signals which are transferred between the two, i.e., the upstream and downstream terminal nodes to switch the virtual paths increases, and accordingly, the virtual paths take a long time to recover from the faults.
This is because when a plurality of virtual paths share the upstream or downstream terminal node, communication of control signals for switching the virtual paths must be performed in correspondence with the number of virtual paths.